Social networking, or social networks, is what we call sites like Facebook, Twitter, Tumblr, Reddit, and so on.
Each user has their own profile page or identity and can link to other users, their pages and their content - think videos, music, pictures, blog entries, forum discussions, notes... Anyone who you're linked to can see your stuff, and vice versa. All this information together creates a newsfeed - a unique home page where you can scroll down to see what everyone else you've linked to (your "network") is doing.
Of course, different social networks might create or display content in different ways. Some, like Facebook, focus more on connecting with people you know. Others, like Reddit, encourage online communities and work more like forums. It's a good idea to find out how each social network works - how to fix your privacy settings, how to report or block other users, and also how to get along with everyone else!
Find out more about different social networks and how to make them work for you here.